The traveling-wave (TW) antenna is a class of ultra-wideband platform-compatible low-profile antennas, including the spiral-mode microstrip (SMM) antennas and miniaturized slow-wave (SW) antenna, among others. The SMM antenna was discussed in publications (Wang, J. J. H. and V. K. Tripp, “Design of Multioctave Spiral-Mode Microstrip Antennas,” IEEE Trans. Ant. Prop., March 1991; and Wang, J. J. H., “The Spiral as a Traveling Wave Structure for Broadband Antenna Applications,” Electromagnetics, 20-40, July-August 2000) and U.S. Pat. No. 5,313,216, issued in 1994; U.S. Pat. No. 5,453,752, issued in 1995; U.S. Pat. No. 5,589,842, issued in 1996; U.S. Pat. No. 5,621,422, issued in 1997; U.S. Pat. No. 7,545,335 B1, issued in 2009) which are incorporated herein by reference. The SW antenna is a subset of the TW antenna with its size miniaturized by the SW technique (U.S. Pat. No. 6,137,453 issued in 2000, which is incorporated herein by reference). These thin planar antennas generally consist of an ultra-wideband planar radiator in the form of a multi-arm spiral, sinuous structure, or other frequency-independent geometries, among which the most widely used is the two-arm spiral antenna, having a unidirectional radiation pattern. The planar multi-arm spirals generally take an Archimedean or equiangular form, as widely discussed in the literature and in particular in the paper by Wang and Tripp (1991) cited above. (pp. 333-334).
The unidirectional radiation pattern is due to mode-1 of TW modes; presence of other TW modes, 0, 2, 3, 4, etc. would distort the radiation pattern. Because of the lack of full symmetry, the commonly used two-arm unidirectional spiral radiator cannot achieve a high degree of mode purity, thus is limited in radiation pattern performance. For applications requiring high-quality radiation patterns, such as the GNSS (Global Navigation Satellite System) receive antenna or elements in planar phased arrays, a four-arm spiral radiator in the SMM antenna was more desirable (e.g., Wang and Triplett, “High-Performance Universal GNSS Antenna Based on GNSS Antenna Technology,” IEEE 2007 International Symposium on Microwave, Antenna, Propagation and EMC Technologies for Wireless Communications, Hangzhou, China, 14-17 Aug. 2007 which is incorporated herein by reference).
Unfortunately, to realize the potential of the four-arm SMM antennas, or the cavity-loaded spiral antenna, a high-quality four-terminal feed is needed to provide equal amplitude and relative phases of 0°, 90°, 180°, 270°, respectively. Such a complex feed, which uses a number of hybrids, power dividers, couplers, matrices, etc. leads to enormous escalation in cost and reduction in gain/efficiency as compared with the two-arm version. Additionally, the complexity and size of such a four-arm feed pose a serious difficulty in its physical implementation in GNSS and array antennas.
Disclosed are various embodiments for a method in which these 4-arm unidirectional TW antennas are fed with a mechanism using a single balun that is generally smaller, much simpler, and thus much less costly, feed. The geometric symmetry of the new approach can also lead to a more accurate feed and thus improve the high performance of the four-arm version further above the two-arm version, at a low cost.